During the etching operation in the fabrication of semiconductor devices, the semiconductor wafer to be etched is placed in a plasma etching chamber, which is typically a semi-closed fabrication chamber with the continuous flow of one or more gaseous components therethrough via a vacuum pump. To facilitate the even distributions of the flowing gas, a baffle plate, or exhaust plate, is horizontally installed inside the etching chamber, dividing the fabrication chamber into an upper chamber and a lower chamber.
FIG. 1A shows an illustrative schematic cross-sectional view of a conventional plasma etching chamber. The chamber 11 is divided by a horizontally disposed baffle plate (or exhaust plate) 12 into an upper chamber 13 and a lower chamber 14. The fabrication chamber 11 also includes a cathode 15, a focusing ring 16, and an electrostatic chuck (ESC) 17 upon which a wafer 18 is disposed. The plasma etching chamber 11 is enclosed by a chamber wall 19.
FIG. 1A also shows that a turbo pump 22 and a throttle valve 23 are used to pump one or more gases into the fabrication chamber 11. FIG. 1B is a top view of a conventional baffle plate 12. Typically, the baffle plate 12 is a ring-shaped plate having a plurality of radially extending perforations 24. The baffle plate 12 is typically separated from the cathode 15 by an insulating ring 21.
To control the pressure inside the fabrication chamber and the flow of gas into the chamber, a pressure sensor 25 is often installed. The pressure sensor 25 is connected to the throttle valve 23 via a CPU, not shown. Analog signals obtained from the pressure sensor 25 are converted into digital signals by a D/A converter, which are then compared against a predetermined value stored in the CPU. The CPU then sends control signals to the throttle valve so that appropriate adjustments can be made to maintain the gas pressure inside the fabrication chamber at or close to the predetermined value.
Typically the pressure sensor 25 is a capacitance-type pressure sensor which comprises a thin membrane attached to a capacitance circuit. Once a pressure difference is present between the pressure chamber and the manometer, which typically has been calibrated at the factory, the sensitive thin membrane would move, causing the capacitance of the sensor circuit to change accordingly. The tiny change in the circuit capacitance is then converted to pressure readings.
Due to the sensitive electronics used in the pressure sensor, the pressure sensor is usually placed away from the etching chamber, as shown in FIG. 1A. If it is placed inside or near the plasma chamber, the sensing electronic circuit would be easily subject to electrical interference when the plasma discharge is turned on. In the worst case, the sensor may suffer permanent damages. On the other hand, even if perfect grounding and shielding can be provided to prevent damages, interference from charged particles (i.e., ions and electrons) contained in the plasma is inevitable and can cause serious signal stability problems.
Due to concerns over plasma attacks and electronic interferences, the pressure sensor is, as shown in FIG. 1A, placed away from the center of the etching process chamber and usually close to the pump and is separated from the etching chamber by the baffle plate. The main function of the baffle plate is to prevent polymer molecules and other unwanted process by-products, which are generated during the plasma etching process, from entering the pump. These foreign particles can noticeably degrade the quality and even substantially reduce the service life of the expensive pump.
It has been discovered that, after repeated usage of the plasma etching chamber, appreciable amounts of polymer deposits will accumulate on the surface of the baffle plate. With time, at least some the slit holes of the baffle plates will be clogged with the polymer deposits. This creates a restriction to the flow of the etching gas to the vacuum pump. As a result, a pressure difference will exist between the inside and outside of the etching chamber, and the pressure sensor placed outside of the etching chamber will no longer be able to reliably provide the actual pressure reading inside the etching chamber. Depending on the condition of the etching chamber, a pressure discrepancy of about 20 m Torr may be expected at a base pressure of about 40 to 70 m Torr inside the etching chamber. Typically, the clogging problem is not detected until the etching chamber is opened and inspected. Normally, routine inspections may identify and solve most of the clogging problems. However, if certain conditions cause the etching process to proceed abnormally, the polymer deposition may become accelerated and the clogging problem may turn out to be more serious than one can expect.
U.S. Pat. No. 5,694,207 discloses a method for in-situ monitoring of the etch rate in an etching chamber by measure intensity values of radiations at wavelengths of 388.5 and 443.7 nm, and computing the quotient thereof. In order to further overcome the large noise relative to the measured emission intensity, the invention further suggests measuring radiation intensities at four wavelengths. The large noise implied in the '207 patent and the fact that pressure measurement must be based on a relatively invariant species seemed to rule out using optical means to measure the etching pressure.
In order to more accurately monitor the true pressure inside the etching chamber so as to provide more precise control of the etching condition, it is important to develop methods that will overcome the above-mentioned problems. Imprecise pressure control inside the etching chamber can adversely result in significant increases in the product rejection rate, thus, it is critically important that the above-mentioned problem be carefully studied at and that a suitable solution be developed so as to improve the product yield rate. As semiconductor devices are becoming more like a common commodity, the profit margin of fabricating semiconductor devices is constantly decreasing. As a result, it is important to look at every process parameter that may affect the failure or rejection rate of the fabricated products, so as to reduce production cost.